1. Field of the Invention
The field of the invention is that of cockpit displays in which the display devices comprise a liquid crystal matrix, also called “LCD”, standing for “Liquid Crystal Display”. These display screens are nowadays tilted in all types of fixed or revolving wing aircraft in both civilian and military domains.
2. Description of the Prior Art
An LCD screen operates on the principle of matrix addressing. As can be seen in FIG. 1, the LCD screen is made up of coloured individual pixels organized in a matrix. Six pixels P are represented in FIG. 1 with different shades of grey representing the colours of the pixels. Each pixel P is made up of a thin thickness of liquid crystal contained between two electrodes, a first so-called control electrode and a second, back electrode B, called “backplane”, common to all the matrix and symbolized by ellipses in FIG. 1. The potential difference or ddp between the electrodes is controlled by a so-called “TFT” (thin film transistor) transistor. This potential difference acts on the orientation of the liquid crystal molecules and therefore on their light transmission. Each transistor TFT is located at the intersection of a row L and of a control column C. The gate G of each transistor TFT is linked to a particular row and the drain D to a particular column.
A video image generator drives, through electronic circuits called drivers, the transistors TFT located at the intersection of the different rows and of the different columns according to the image to be displayed. The drivers are organized as “row drivers” and as “column drivers”.
The column drivers receive the video information to be displayed which is validated by the row drivers. The column drivers convert the digital information from the video generator into analogue voltages proportional to the voltage references called “GMA”, standing for GamMA voltage. These voltages are applied to the drains D of the transistors TFT.
The row drivers apply to the gates G of the TFTs of one and the same row a line switch-on voltage “VG_on” in order to apply to the liquid crystal the analogue voltages supplied by the column drivers, the other rows being kept switched off by the row switch-off voltage “VG_off”. Once the row is written, the column drivers present the new analogue voltages and the row drivers switch on the next row, and so on for all the rows of the matrix screen.
The LCD screens used are generally based on the so-called “twist nematic” (TN) technology because it offers the advantage of offering more optical transmission than the other technologies.
FIGS. 2a and 2b represent, as a function of the time t, the voltages applied in volts in two configurations of use. In the first, so-called “BLACK LCD”, configuration, the liquid crystal is opaque; in the second, so-called “WHITE LCD”, it is transparent. The “backplane” voltage represented by a dotted line in FIGS. 2a and 2b is continuous and set at around 6 volts. In both cases, in order not to create a marking, the liquid crystal receives an alternately positive then negative DDP corresponding to the “positive phases” and “negative phases” of FIGS. 2a and 2b. 
In the first configuration represented in FIG. 2a, the liquid crystal is opaque if the DDP is greater than 4 volts. The column voltages then vary between 0 and 12 volts, that is to say by ±6 volts around the 6 volt backplane voltage.
In the second configuration, represented in FIG. 2b, the liquid crystal is transparent if the DDP is virtually zero or less than 1 volt. The column voltages then vary between 5 and 7 volts, that is to say ±1 volt around the 6 volt backplane voltage.
The drawback of this technology is that it naturally presents a white screen that is said to be “normally white” when the latter is defective or uncontrolled. This white screen with strong brightness compared to a traditional display with dark background of the “PFD” (primary flight display) can be a nuisance to the pilot.
FIG. 3 represents a set of two dual displays denoted LCD1 and LCD2. The expression “dual display” should be understood to mean a display comprising a single LCD matrix arranged in such a way as to display two totally independent or segregated images. More specifically, a dual display comprises two independent power supplies, two independent light boxes, two independent graphic generations and two independent sets of row and column drivers. Thus, a first failure of any kind does not result in the total loss of the display device. In FIG. 3, the left hand display LCD1 is perfectly functional and presents two displays denoted D1G and D1D. The left hand part D2G of the right hand display LCD2 has failed and displays a white screen. Only the right hand part D2D is operational.
So as to restore a black screen, the failure of this type of screen results either in the automatic switching off of the light box which lights the matrix so as to restore a black screen, or the manual switching off of the equipment by the pilot. This switching off results in the complete loss of the equipment. This outcome is highly detrimental in the case of a dual display presenting two different and independent display types because it causes the loss of the second display, thus disrupting the screen reconfigurations needed to satisfy the aircraft certification and safety constraints.